The invention relates generally to a thin film semiconductor device and more particularly to a device including an active silicon thin film having particularly desirable characteristics for use in a thin film transistor that can be included as an active element in a liquid crystal display device.
It is becoming important to construct liquid crystal displays having a large screen size and high resolution so that a large volume of information can be displayed. Consequently, it is becoming important to improve systems for driving the display in order to maintain acceptable display quality. This has led to wide spread use of active matrix display systems that include a switching transistor at each picture element (pixel) of the display. This allows high quality liquid crystal displays to be formed with hundreds of thousands of picture elements.
Conventional active matrix displays are constructed with transparent substrates and a matrix of switching transistors, such as thin film transistors, on the substrate. Transparent insulating substrates for transparent displays include melt quartz plates, glass plates and the like. It is desirable to form the transparent substrate from low cost ordinary glass in order to increase the size of a liquid crystal display screen without undesirably increasing the cost of the screen, or when it is desired to reduce the cost of a screen without decreasing its size. However, conventional techniques for forming acceptable quality thin film transistors on an ordinary glass substrate to form a liquid crystal active matrix display having suitable properties have been unsatisfactory.
Amorphous silicon and polycrystalline silicon are commonly used for forming an active semiconductor layer for a thin film transistor. Polycrystalline silicon which exhibits high operating speed is useful for forming a thin film transistor which has a driving circuit formed integrally therewith. However, in order to form a silicon thin film on an ordinary glass substrate, the silicon film must be formed at a low temperature of about 600.degree. C. or less. A technique for accomplishing this would be extremely useful and can also be applied to integration of SRAM's or the like and formation of multi-layer LSI's.
One attempt at forming a high quality silicon thin film at a low temperature is low pressure chemical vapor deposition (LPCVD). However, conventional LPCVD processes can only form silicon thin films having unsuitable quality. For example, such films tend to have unsuitably low crystallinity and as well as mainly {220} preferred orientation when deposited at 600.degree. C. or less. Such low pressure CVD polycrystalline silicon thin films are unsuitable for many semiconductor devices.
Improved silicon thin films have been formed by depositing the thin film at as high a deposition temperature as possible (610.degree. to 640.degree. C.) and at a deposition pressure of 40 mTorr to 750 mTorr. Such a method is discussed in Appl. Phys. Lett., 50(26), p. 1894 (1987). Another method involves depositing a silicon film on an insulating substrate by low pressure CVD at a temperature of 570.degree. C. or less and then heating the film to a temperature of about 640.degree. C. or less for 24 hours to change the orientation and crystallinity of the film. This method is discussed in Japanese Patent Laid Open No. 63-307776. Another example of a method of forming improved thin films is to deposit an amorphous silicon thin film at a temperature of about 300.degree. C. or less by an RF magnetron sputtering process or by a plasma CVD process and then apply laser beams to the film. Such a method is discussed in Jpn. J. Appl. Phys., 28, p. 1871 (1989) and in Technical Research Report of the Society of Electron Information Communication EID-88-58.
Despite having some advantages, the aforementioned processes have not proved to be fully satisfactory. Heating the thin film or applying laser beams to the film adds complicated and tedious production steps compared to low pressure CVD. This reduces productivity and necessitates purchasing expensive processing equipment and increases production costs.
The principal drawback of depositing an acceptable quality silicon thin film to serve as an active layer of a semiconductor device by conventional low pressure CVD is that the deposition temperature is too high. For example, if a thin film device is to be formed on a low-cost glass substrate, the highest permissible temperature for the production process is about 600.degree. C. Furthermore, the substrate can only be kept at that temperature for several hours. In addition, when a thin film semiconductor device is used for a three dimensional LSI, SRAM or the like, it is beneficial to produce the semiconductor device at 600.degree. C. or less to protect lower layer transistors and connections. However, conventional low pressure CVD forms silicon thin films having undesirably low crystallinity and having mainly {220} preferred orientation. Polycrystalline silicon thin films formed by conventional low pressure CVD are not suitable for active layers of semiconductor devices. Thus, they should not be used as switching elements for liquid active matrix crystal displays, nor should they be used for three dimensional LSI's, SRAM's and the like.
Accordingly, it is desirable to provide a high quality silicon thin film for a thin film switching device which overcomes the inadequacies of the prior art.